Depicted in FIG. 1 is an ink jet colour printer on which the main parts are labelled as follows: a fixed structure 41, a scanning carriage 42, an encoder 44 and a variable number of printheads 40 which may be either monochromatic or colour.
The printer may be a stand-alone product, or be part of a photocopier, of a plotter, of a facsimile machine, of a machine for the reproduction of photographs and the like. The printing is effected on a physical medium 46, normally consisting of a sheet of paper, or a sheet of plastic, fabric or similar.
Also shown in FIG. 1 are the axes of reference:
x axis: horizontal, i.e. parallel to the scanning direction of the carriage 42; y axis: vertical, i.e. parallel to the direction of motion of the medium 46 during the line feed function; z axis: perpendicular to the x and y axes, i.e. substantially parallel to the direction of emission of the droplets of ink.
FIG. 2 shows an axonometric view of the printhead 40 according to the known art, on which nozzles 56, generally arranged in two columns parallel to the y axis, and a nozzle plate 106 are indicated.
The composition and general mode of operation of a printhead according to the thermal type technology, and of the “top-shooter” type in particular, i.e. those that emit the ink droplets in a direction perpendicular to the actuating assembly, are already widely known in the sector art, and will not therefore be discussed in detail herein, this description instead dwelling more fully on only those features of the heads and the head manufacturing process of relevance for the purposes of understanding this invention.
The current technological trend in ink jet printheads is to produce a large number of nozzles per head (≧300), a high definition (≧600 dpi), a high working frequency (≧10 kHz) and smaller droplets (≦10 pl) than those produced in earlier technologies.
Requirements such as these make it necessary to produce actuators and hydraulic circuits of increasingly smaller dimensions, greater levels of precision, and strict assembly tolerances. They also exasperate the problems generated by the different coefficients of thermal expansion among the different materials the head is made of.
Greater reliability is also required of the heads, especially where there is allowance for interchangeability of the ink tank: the service life of these heads, called semifixed refill heads, is close to that of the printers.
Thus there is a need to develop and produce fully integrated monolithic heads, in which the ink ducts, the selection microelectronics, the resistors and the nozzles are integrated in the “wafer”.
Achievement of a result such as this is furthered by the small dimensions of the drops, now of volumes less than 10 pl (pl=picolitre), and which require actuation energies of less than 3 μj (μj=microjoule) per actuator.
Numerous solutions for producing heads with a monolithic actuator have been proposed, such as for instance the one described in the Italian patent application TO 99A 000610 “Monolithic Printhead and Associated Manufacturing Process”.
FIG. 3 depicts, by means of an axonometric view and a cross-section, a monolithic actuator 80 comprising:                a die 61 of semiconductor material, generally silicon;        a structure 75 made of a layer of, for instance, polyamide or epoxy resin, having thickness preferably between 20 and 50 μm;        the nozzles 56 arranged in two columns parallel to the y axis.        
In the same figure, in an enlarged section AA, parallel to the plane z-x, the following may be seen:                chambers 57, arranged in two columns parallel to the y axis;        ducts 53;        a substrate 140 of silicon P;        a groove 45, having its greater dimension parallel to the y axis, and accordingly also to the columns of nozzles 56;        a lamina 64, which in turn comprises:        a diffuse layer 36 of N-well silicon        an insulating layer 35 of LOCOS SiO2;        a resistor 27 of tantalum/aluminum having a thickness of between 800 and 1200 Å;        a layer 34 of polycrystalline silicon;        a contact 37 of N+ silicon;        an “interlayer” 33 of BPSG;        an “interlayer” 32, consisting of a layer of TEOS SiO2;        a layer 30 of Si3N4 and SiC for protection of the resistors;        channels 67;        an anti-cavitation layer 26, made of a layer of tantalum covered by a layer of gold;        ink 142; and        a droplet of ink 51.        
According to the patent application cited, the groove 45 is produced in part in a “dry etching” step and in part in a “wet etching” step, both known to those acquainted with the sector art. The wet etching proceeds according to geometrical planes defined by the crystallographic axes of the silicon, which set the orientation of the groove 45 along the x-y plane. To be able to produce the columns of nozzles 56 parallel to the groove 45, there is therefore the need to dispose of references accurately aligned to the crystallographic axes of the silicon: with the aid of FIGS. 4 and 5, a procedure commonly followed for this purpose is described.
A circular shaped wafer 66 commonly has a reference 65, called “flat” by those acquainted with the sector art, oriented perpendicularly to one of the crystallographic axes of the silicon, with an error angle ε generally contained within ±1°. A geometric reference 63 is constructed perpendicular to the flat 65. The groove 45, etched in a wet process, will on the other hand be parallel to the crystallographic axis of the silicon, and thus rotated by the angle ε with respect to the geometric reference 63. If the columns of nozzles 56 were oriented parallel to the geometric reference 63, they would not be parallel to the groove 45, thereby compromising operativity of the head.
This makes it necessary to construct a crystallographic reference 62 (FIG. 5) which is parallel to the actual crystallographic axis of the silicon. One way of constructing such a reference is described, for example, in the article “Alignment of Mask Patterns to Crystal Orientation” by G. Ensell presented to the 8th International Conference On Solid-State Sensors and Actuators, Stockholm, Jun. 25-29, 1995.
To this end, various test notches 55 are etched, of circular shape and arranged according to an arc of a circle with centre C. Then a wet etching is performed which, local to each notch, produces a square-shape subetching having sides parallel to the crystallographic axes of the silicon. Generally the sides of the subetchings of two notches, indicated with a and b, happen to belong to one and the same straight line: the crystallographic axis sought is perpendicular to the radius r which joins a median point between a and b with C, and becomes visible when the crystallographic reference 62 is plotted, parallel to which the columns of the resistors 27 and of the corresponding nozzles 56 are aligned.
The process described enables to reduce the error angle ε for example to within ±0.1°, but is highly complex. It also requires that the mask defining the groove, which is necessarily on the face of the wafer that contains the crystallographic reference 62, be aligned to the masks which define the other parts of the actuator, which are on the opposite side of the wafer.
Methods have therefore been proposed by means of which it is possible to etch the groove 45 in such a way that the latter aligns automatically to a desired reference, such as for instance to the columns of the nozzles 56, even if the crystallographic axis of the silicon has a slightly different orientation. One of these methods is described for instance in the article “A Thermal Inkjet Printhead with a Monolithically Fabricated Nozzle Plate and Self-Aligned Ink Feed Hole” published in the Journal of Microelectromechanical Systems, Vol. 8, No. 3, September 1999, and is herein described summarily with the aid of FIG. 6, where a wafer of semiconductor material is depicted in section. The following are labelled:                a substrate 140 of silicon P;        an insulating layer 35 of LOCOS SiO2;        a metallic layer 71, made for instance of Au;        a contact 37 of silicon P+ having the purpose of improving the electrical connection between the metallic layer 71 and the substrate 140 of silicon P;        an N diffusion, 38        an electrolyte 82; and        a cathode 81, made of a conducting material resistant to the electrolyte 82, of platinum for instance.        
On applying a voltage V between the cathode 81 and the metallic layer 71 a current field flows, indicated by the field lines 52, which assumes a shape defined with precision by the geometry of the insulating layer 35 of LOCOS SiO2 and by the silicon P+ contact 37. The substrate 140 of silicon P is etched electrochemically local to the field lines 52 until the metallic layer 71 is reached. In this way the electrochemical grooves 68 are made (FIG. 7a) which, in the vicinity of the metallic layer 71, assume the shape and orientation defined with precision by the geometry of the insulating layer 35 and by the silicon P+ contact 37, totally independent of the orientation of the crystallographic axis of the silicon.
The electrochemical etching also has the advantage of being fast (from 20 to 30 μm per minute), much faster than wet anisotropic etching (from 0.5 to 1 μm per minute) and ICP dry etching (from 5 to 10 μm per minute).
The electrochemical grooves 68, however, have extremely rounded edges which increase their length on the side facing the cathode 81, which will be turned towards the ink tank during operation: when the different grooves 68 are close together, as is the case in colour heads with a large number of nozzles, the silicon between them is excessively diminished, and no longer has a flat surface coplanar with the edges of the die, rendering a subsequent sealing operation difficult. Also in a monochromatic head, which has a single groove as can be seen in FIG. 7b, the edges of the die are rounded rendering the sealing operation difficult.